ALBERT

Description: Yield estimation of small explosions at local distances represents a challenge for the nuclear explosion monitoring community. We have examined the feasibility of using short-period surface-wave magnitudes, called , to estimate explosion yields at local distances (〈100 km). We have modified the Russell (2006) M s formula, which was derived for periods of 8–25 s for distances beyond 50 km, for application at local distances and 1 s period. We have studied short-period surface-wave attenuation in diverse lithologies in order to incorporate an attenuation term in the magnitude scale, which is suitable for . We have also incorporated multiple excitation corrections for based on the near-source seismic velocities, which greatly affect the source-region amplitudes for . It is important to note that in the formula the excitation is estimated from the measured group velocity. We have also derived a new Butterworth filter cutout definition for filtering near 1 s period at distances between 2 and 100 km. We used the new formula to estimate for 39 small (37≤ Y ≤12,270 kg TNT equivalent) and shallow (〈120 m) explosions detonated in North America in lithologies ranging from alluvium to granite. Regressions of the magnitudes with yield result in the equation for chemical explosions with Y 〈12,270 kg. An F factor with 95% confidence was determined to be 2.25, giving lower and upper bounds on the yield estimates of Y /2.25 and Y x 2.25, respectively. We applied the relationship (assuming factor of 2 equivalence between chemical and nuclear) to nuclear explosions detonated at the Degelen and Shagan, Kazakhstan, test sites. The estimated yields based on magnitudes were often within 20% of the true yield and had smaller F factor than the estimated yields for United States chemical explosions. Online Material: Tables of event information and estimates.

Description: We studied seismic-wave generation from five small (60–122 kg) fully contained explosions detonated in Barre Granite as a part of the New England Damage Experiment (NEDE). The explosions were conducted using three types of explosives with different velocities of detonation (VOD): black powder, ammonium nitrate fuel oil (ANFO) emulsion, and composition B (COMP-B). Empirical evidence suggests that the low VOD explosives produce more shear-wave energy than high VOD explosives. The proposed mechanisms to explain this effect include: (a) inhibition of gas-driven fracture propagation by thicker pulverized zone for high VOD explosions, and (b) fracture toughness increase at higher loading rate. The main objective of the experiment was to study differences in shear-wave generation between different types of explosives, and to determine the likely mechanism responsible for these differences. Seismic amplitude analysis revealed that COMP-B releases more energy and larger amplitude P waves for the same weight of explosives, while producing smaller amplitude S waves. Furthermore, large radial cracks were observed on the surface after the ANFO and black powder shots, while there was no surface fracturing after the COMP-B shots. Thus, longer fractures correlate with higher S -wave amplitudes. However, drilling into the source region indicates that high VOD explosions may actually produce a smaller pulverized zone, which means that fracture inhibition is a less plausible explanation. Therefore we hypothesize that increase in loading rate, in combination with shorter impulse duration for high VOD explosives, inhibits fracture processes, and subsequently reduced S -wave radiation.

Description: Improved understanding of the seismic radiation generated by explosions in low coupling (damaged/fractured) media is extremely important for nuclear monitoring, as source coupling affects both detection and yield estimation. Some empirical evidence for seismic amplitude reductions have been noted for nuclear and chemical explosions detonated in fractured media (e.g., Sokolova, 2008 ). In order to define the physical mechanism responsible for the amplitude reduction and quantify the degree of the amplitude reduction in fractured rocks, we conducted Phase I of a multi-phase explosion experiment in central New Hampshire. The experiment involved conducting explosions of various yields, including a 46.3-kg explosion in the damage/fracture zone of a 231.8-kg explosion and a 46.8-kg shot in nearby undamaged rock. Our analysis confirms a seismic amplitude reduction in damaged rock by a factor of 2–3. The amplitude differences are frequency dependent, with the explosion in the undamaged rock having a higher corner frequency than the explosion in the damaged zone. The overshoot parameter for the virgin/undamaged rock shots is higher than that for the damaged rock shot. We found that the corner frequency correlates with the overshoot parameter, and only weakly correlates with the yield. Additional experiments will be conducted in the near future to further quantify seismic-wave characteristics as a function of the depth of burial, type of explosives, and other factors. Online Material: Movies of explosions.

Description: We studied seismic body-wave generation from four fully contained explosions of approximately the same yields (68 kg of TNT equivalent, where TNT stands for trinitrotoluene) conducted in homogeneous granite in Barre, Vermont. The explosions were detonated using three types of explosives with different velocities of detonation: black powder (BP), ammonium nitrate fuel oil/emulsion (ANFO), and composition B (COMP B). The main objective of the experiment was to study differences in seismic-wave generation among different types of explosives and to determine the mechanism responsible for these differences. The explosives with slow burn rate (BP) produced lower P -wave amplitude and corner frequency, which resulted in lower seismic efficiency (0.21%) in comparison with high burn rate explosives (1.3% for ANFO and 1.9% for COMP B). The seismic efficiency estimates for ANFO and COMP B agree with previous estimates for nuclear explosions. The body-wave radiation pattern is consistent with an isotropic explosion with an added azimuthal component caused by vertical tensile fractures oriented along pre-existing microfracturing in the granite, although the complexities in the P - and S -wave radiation patterns suggest that more than one fracture orientation could be responsible for their generation. Analysis of the S / P amplitude ratios suggests that a significant fraction of the shear-wave energy can be explained by opening of the tensile fractures and spall.

Description: Love waves have the potential to aid in discrimination for anomalous explosion events. We develop a calibrated mathematical formulation for an explosion discriminant that combines Rayleigh- and Love-wave magnitude values and employs an error model that correctly partitions variances among events and stations separately. The discriminant is calibrated using a global data set of 124 earthquakes and 26 nuclear explosions and applied to the May 2009 Democratic Republic of North Korea (DPRK) announced nuclear test, as well as the calibration data set. All 26 explosions were correctly identified; only 6 earthquakes were incorrectly identified as explosions. Compared to an analogous treatment using only Rayleigh data, the combined discriminant improves the DPRK event p -value only nominally but reduces the number of false positives in the calibration data set by 70%, with no additional false negatives. While not dramatically improving the discrimination power for anomalous events, such as the 2009 DPRK test, the combined discriminant proposed here offers improved screening capabilities for typical events. Online Material: Earthquake and explosion calibration data set.

Description: The May 2012 HUMBLE REDWOOD III (HRIII) experiment series in New Mexico provides a unique dataset to study surface-wave generation from explosions conducted above and underground for different rock types. Four 90.6 kg trinitrotoluene-equivalent explosions were detonated either at 2 m height-of-burst (HOB) or 7 m depth-of-burial (DOB) at separate alluvium and limestone test sites. For the alluvium site, data from a temporary seismoacoustic network show that fundamental-mode surface waves ( ) from the 7 m DOB in-alluvium shot were four to five times larger than the above-alluvium shot. The amplitudes from the 7 m DOB limestone shot were 15 times larger than recorded from the collocated 2 m HOB shot. To model these differences in , we generated 1D velocity models for both test sites using observed surface-wave dispersion. We considered two different methods for synthetic seismogram generation. For the aboveground shots, we have coupled near-field blast wave pressures and shapes with source medium properties to model seismic data at distance. For the underground shots, we use explosion source theory to estimate a moment for scaling explosion synthetics. For both above and underground shots, the synthetics provide excellent fits to the observed 1–5 Hz data. This modeling provides a viable technique to predict peak particle velocities for surface and aboveground explosions in different rock types that can be used to estimate combined seismoacoustic yields. Online Material: Movies of four explosions studied in this paper.

Description: A series of small chemical explosions were conducted in May 2012 at Kirtland Air Force Base, New Mexico, as a part of the HUMBLE REDWOOD III experiments. Two charges of 111 kg of ammonium nitrate and fuel oil were detonated at 7 m depth in limestone. The second explosion was detonated in a small asymmetric cavity produced by the first explosion at the same working point as the first explosion. Light Detection and Ranging (LiDAR) was used to image the cavity and to determine an equivalent spherical radius of 0.82 m. The seismic amplitudes for the cavity shot were reduced by a factor of 3–4 compared to the confined shot in limestone. In this article, we quantify the generation of the small cavity and the seismic decoupling it produced.

Description: A mine blast is typically composed of many individual explosions detonated in a temporal and spatial grid, a process called delay firing. A recent experiment at a granite quarry in Massachusetts included detonation of a delay-fired (DF) mining blast within 250 m of a single-fired (SF) explosion. The DF blast consisted of 48 holes detonated over 0.495 s. It produced longer duration Rayleigh waves with envelope functions that peaked later than Rayleigh waves generated by the SF explosion. Measured group velocity dispersion curves for recordings of the DF blast were slower than the estimates for the SF explosion, and with larger differences noted at near-source stations. Using a formula derived initially by Ben-Menahem (1961) for dynamically rupturing fault lines, we correct the dispersion curves using an initial group delay of 0.32 s. We demonstrate that for delay-firing patterns of ~0.5 s, the effect of the initial group delay is not significant beyond 25 km; however, for longer-duration blasts, the effect could be observed at distances beyond 100 km. Mine blast firing patterns should be considered prior to using the resulting dispersion curves for crustal structure determination. Online Material: MPEG movies of the DF and SF shots.

Description: The January 2011 calibration explosions at the Sayarim Military Range in the Negev Desert, Israel, provide a unique dataset to study seismic and acoustic partitioning for surface sources at near-source and local distances. We present an analysis of the seismic and overpressure/acoustic signals generated from two explosions, which included 10,240 and 102,080 kg of mainly ANFO with some Composition B, detonated on the surface, on 24 and 26 January, respectively, at different times of the day. A temporary seismo-acoustic-overpressure network was deployed at distances between 0.1 and 39 km to supplement data from permanent stations in Israel. The near-source data confirm the explosions produced overpressure signals consistent with complete and simultaneous surface detonation of 7.4 and 76.8 metric tons of TNT-equivalent explosives. We observe acoustic amplification at local distances (〉5 km) south of the larger explosion that can be explained by a low-altitude southward flowing wind at the detonation time. For the smaller shot, the southward flow was not present and amplification was not observed in the pressure data. We present results of modeling these overpressure data with blast pressure scaling models with and without meteorological data. Seismic phases generated by the surface shots include P waves, fundamental and higher mode Rayleigh waves, and Love waves that appear to have been generated at or very near the explosion source. The seismic ground motion is less than would be expected for fully coupled explosions of 7.6 and 76.8 tons, and at near-source distances (〈10 km) can be modeled well by reducing ground-motion predictions from the Fuis et al. (2001) model by a factor of 4. The results from the Sayarim explosions, combined with past and future experimental datasets, will lead to an improved understanding of the seismo-acoustic source function for explosions and energy partitioning between seismic and acoustic waves.

Description: The traditional M s : m b discrimination method is routinely used for distinguishing between earthquakes and explosions within dense networks, but there is a need to improve discrimination at smaller magnitudes; therefore, we need magnitude scales that can successfully be applied to data from sparse networks. We developed a unified Rayleigh- and Love-wave magnitude scale ( M s U) that is designed to maximize available information from single stations and then combine magnitude estimates into network averages. By combining Love- and Rayleigh-wave amplitudes, we minimize the effect of earthquake radiation patterns from sparse networks, thereby improving discrimination between earthquakes and explosions. M s U is built from M s ( V MAX ) ( Russell, 2006 ) and is calculated from Love and Rayleigh waves that are narrowband filtered and corrected for propagation and source effects at periods between 8 and 25 s to find filter bands of maximum energy propagation. The data are also corrected for censoring effects at the station level, because either Rayleigh or Love waves may be below the signal-to-noise ratio threshold at a given period. We applied M s U to 39 earthquakes (3.21〈 M w 〈5.08) located in the Yellow Sea and Korean Peninsula region, as well as to the three North Korean nuclear tests (4.1〈 m b 〈5.1). By using M s U: m b as a discriminant, there is an increase in the separation of small magnitude earthquakes and explosions in sparse networks and a significant reduction in outliers, as shown in the application from the Korean Peninsula. This research addresses the theory, methods, and capability of M s U as a discriminant. Online Material: Detailed spectral analysis and M s U censoring algorithm, and figures of filter specifications.